One objective is to examine in conscious dogs the respective roles of the carotid baroreceptors, the cardiopulmonary receptors, the peripheral sympathetic nerves and the cardiac autonomic nerves in the distribution of cardiac output and maintenance of arterial blood pressure during the stress of exercise and of upright posture: to define the time available for, and the determinants of ventricular filling, and why this is so drastically shortened by anesthesia and thoracotomy: to determine why the resistance vessels in exercising muscle have a depressed response to sympathetic nerve stimulation in the first minutes of exercise. The second objective is to determine if the carotid baro and chemoreceptors and the somatic muscle afferents behave like the cardiopulmonary receptors in altering the release of renin; and if so, what is the time couse of release, and is it primarily a neural response or secondary to changes in renal hemodynamics; to determine if the carotid and cardiopulmonary mechanoreceptors interact in the release of renin in conscious dogs; to examine the mechanisms causing augmentation of the vascular response to interruption of cardiopulmonary inhibitory traffic when that of the carotid sinus is withdrawn; to define the major cardiac pathways of vagal inhibitory fibers from the left heart.